Severe attacks of asthma can occur without apparent warning, especially at night, and may cause death. Adolescents appear to be at increased risk for this. We studied 11 children with asthma twice to determine if diurnal variation occurred in airways reactivity to methacholine and if variability was constant. Spirometry and PC20 were obtained at 1600 and 0400 hr and spirometry alone at 2100 hr. Psychological tests measuring personality, behavior, anxiety and depression were done at each study. 4/11 had more than 2-fold greater reactivity at 0400 then 1600; this occurred only on the first study. The remaining 7 had no change or improvement in PC20 at 0400. The 4 with variability tended to have more severe asthma, compared to the 7 without variability. For the 4 studies with variability FEV13 at 2100 were not different than at 1600, so did not predict the results. While anxiety with the testing might account for greater reactivity at 0400 hr only on the first study, this was not detected possibly because the psych tests were insensitive to testing anxiety. The 4 children with variability were not different with regards to sexual maturation or psychological status. While the numbers in this study are small, our results suggest that there is a subset of adolescents with diurnal variability in airways reactivity which may place them at risk for severe sudden asthma during the night.